<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>flex</title>
    <style>
        html,body{
            height: 100%;
        }
        .d1{
            display: flex;
            /* flex-direction: row; */
            justify-content: space-evenly;
            align-items: center;
            height: 100%;
        }
        .sub1{
            width: 200px;
            height: 200px;
            background-color: red;
        }
        .sub2{
            width: 200px;
            height: 200px;
            background-color: blue;
        }
        .sub3{
            width: 200px;
            height: 200px;
            background-color: yellow;
        }
    </style>
</head>
<body>
    <div class="d1">
        <div class="sub1"></div>
        <div class="sub2"></div>
        <div class="sub3"></div>
    </div>
</body>
</html>